Polyolefins with low crystallinity are commercially significant products due to their toughness and flexibility. Since these products are often copolymers of propylene with ethylene and (less frequently) other olefins, such as C4-C12 alpha olefins, styrene, etc., they also have extended low-temperature application range due to the reduced glass transition temperature imparted by the comonomer, especially ethylene. These low-crystallinity polyolefins are often used in combination with crystalline components, such as polypropylene or polyethylene, to fine-tune the stiffness-toughness balance and to extend the high-temperature application range. Changing blend compositions and blend ratios can yield a virtually infinite number of polyolefin blends with a wide range of properties enabling the fine-tuning of those blends to fit the desired application demands while also allowing the production of those finished goods at low cost. These blends are mostly made by compounders, often small operations, that purchase the copolymer and homopolymer components from large petrochemical companies. These compounders prefer using pelletized polymer blend components because they are easy to store, move, and dose.
While these low crystallinity polyolefin products are valued for their ability to increase elasticity, toughness and flexibility, they tend to have both a low melting onset and a low peak melting temperature. They are often nearly or completely amorphous. In addition, these products are often quite soft and exhibit low moduli. These low-crystallinity polyolefins are often difficult to pelletize and the resultant pellets tend to stick together rendering them unusable in compounding operations set up for pelletized feed stocks.
The property capturing the ability of polymer pellets to stay free-flowing during shipping and storage is referred to as “pellet stability”. Pellet stability is an important attribute of polyolefins since good pellet stability affords low-cost shipping, storage, and dosage in the production of polymer blends with tailored stiffness-toughness balance ultimately lowering the cost of producing useful goods. Recognizing the importance of delivering polymers in stable, industry-standard pellet form, petrochemical companies have been developing methods for stabilizing otherwise unstable low crystallinity polyolefins. These methods include dusting the freshly-produced pellets with inorganic compounds, like talc, or by blending a small quantity of off-line-produced, more crystalline component with the base soft polyolefin that “hardens” the pellets to withstand shipping and storage without clumping together. These methods have several drawbacks. Using inorganic powders may introduce an undesired component while providing only limited increase in pellet stability. Melt-blending of off-line-produced stabilizer components tends to be expensive due to the complexity of blending two highly viscous molten polymer streams.
In-line blending through the use of series reactors producing a low crystallinity and high crystallinity component is also known in the art. Utilizing a series reactor configuration, product blending may be accomplished in the solution polymerization reactor itself when the effluent of the first solution polymerization reactor is fed into the second reactor operating at different conditions with optionally different catalyst and monomer feed composition. Referring to the two-stage series reactor configuration of FIG. 1 (prior art), the two different polymers made in the first and second reactor stages are blended in the second stage yielding a blended polymer product leaving the second reactor. Such reactor series configuration may be further expanded into more than a two-stage series configuration (three or more reactors in series). While mixing in the downstream reactor(s) provides good product mixing, particularly when the reactors are equipped with mixing devices, e.g., mechanical stirrers, such series reactor configuration and operation presents a number of practical process and product quality control problems due to the close coupling of the reactors in the cascade. One of the most important difficulties in commercial practice in producing pellet-stable blends is ensuring proper blend and monomer ratios of the soft polymer component and the more stable highly crystalline polymer component to deliver consistent blend quality. Additional complications arise when the blend components have different monomer compositions, particularly when they have different monomer pools, such as in the case of blending different copolymers or in the case of blending homo- and copolymers. Since the monomer streams are blended, there is no option for their separate recovery and recycle mandating costly monomer separations in the monomer recycle lines.
Applying parallel reactors can overcome the disadvantages related to the direct coupling of the polymerization reactors in an in-line polymer blending applying series reactors. While production flexibility is increased, a parallel reactor arrangment necessitates the installation of blending vessels increasing the cost of the process. U.S. Patent Publication No. 2006/0183861 discloses blends of at least two polymers incorporating propylene-derived units and processes for producing such blends wherein one polymer of the blend is a low-crystallinity polymer including propylene-derived units and the second polymer is a high-crystallinity polymer including propylene-derived units. The processes for producing such blends include both series polymerization and parallel polymerizations under solution polymerization conditions. The polymer blends exhibit a reduced tendency for the polymer pellets to agglomerate while maintaining the desirable physical properties, such as elastomeric properties, exhibited by low crystallinity propylene polymers. The solution polymerization processes have limitations in producing highly crystalline, high molecular weight (high MW) products with higher melting point. In particular, the solution process cannot typically produce high MW products that also have high melting point. This limitation makes it less than optimal for making pellet-stable polyolefin blends wherein incorporating highly crystalline (high melting point, i.e., >145° C., or >150° C., or >152° C., or >154° C., or >155° C., or >156° C.) high MW (>75 kg/mol, or >100 kg/mol), or >125 kg/mol, or >135 kg/mol, or >150 kg/mol) polymer component is advantageous. U.S. Patent Publication No. 2006/0183861 is herein incorporated by reference in its entirety.
A need thus exists for an improved and cost-effective method of in-line blending low-crystallinity and high-crystallinity polyolefin components to make pellet-stable polyolefins. More particularly, a need exists for an improved in-line method of blending low-crystallinity and highcrystallinity polyolefin components, wherein the highly crystalline polyolefin component may be made in a homogeneous fluid state enabling in-line blending without fouling the reactor, and the residence time, monomer composition, catalyst choice, and catalyst concentration can be independently controlled in each polymer reactor prior to the in-line blending step.